1. Field of the Invention
Priority is claimed on Japanese Patent Application No. 2006-261678, filed Sep. 27, 2006, the contents of which are incorporated herein by reference.
The present invention relates to an optical scanner which performs scans by the scanning of an optical beam, and, in particular, to an optical scanning device having a structure in which a micro mirror which is supported by torsion bars is made to oscillate so as to cause the direction of an optical beam to change.
2. Background Art
In recent years, optical scanners which scan optical beams of laser light or the like have been used as optical instruments such as bar code readers, laser printers, and head mounted displays, or as the optical capturing devices of input devices such as infrared cameras and the like. Optical scanners having a structure in which a micro mirror obtained via silicon micromachining technology is oscillated have been proposed for this type of optical scanner. For example, the structure described in Japanese Unexamined Patent Application, First Publication No. H07-65098 (Patent document 1) is known (referred to below as ‘Conventional technology 1’). As shown in FIG. 21, this optical scanner irradiates light which is emitted from a light source 100 and reflected by a mirror section 101 onto a detection object 102, and then vibrates the mirror section 101 so that the light is scanned in a predetermined direction of a detected object 102, and is provided with two mutually parallel drive sources 103 and 103 which are formed as cantilevered beams with one end respectively thereof formed as a fixed end and which perform bending operations, a linking component 104 which links together the free end sides of the two drive sources 103 and 103, a torsional deformation component 105 which extends from a center portion of the linking component 104, and the mirror section 101 which is provided on this torsional deformation component 105. The center of gravity of the mirror section 101 is made to sit on the torsion center axis of the torsional deformation component. If the two drive sources 103 and 103 are driven, for example, by a bimorph structure on which a piezoelectric material has been adhered, and are vibrated in antiphase, then torsional vibration is induced in the torsional deformation component 105, and the two drive sources are driven at the resonance frequency of the torsional deformation component 105. As a result, it is possible to vibrate the mirror section over a sizable amplitude.
Moreover, as shown in FIG. 22, the scanner described in Japanese Unexamined Patent Application, First Publication No. H04-95917 (Patent document 2, referred to below as ‘Conventional technology 2’) is a scanner in which a mirror surface 11 is formed by a surface of a vibrator 110 having two elastic deformation modes, namely, a bending deformation mode and a torsional deformation mode, and in which this vibrator is vibrated at the respective resonance frequencies of the two modes. Optical beams irradiated towards the mirror surface of the vibrator are reflected by that mirror surface so that the light is scanned in two directions. If the vibrator is vibrated in a single mode, then this scanner becomes a one-dimensional scanning optical scanner.
Moreover, as an optical scanner in which a micro mirror obtained by means of silicon micromachining technology is oscillated, the structure described in Japanese Unexamined Patent Application, First Publication No. H10-197819 (Patent document 3) is known (referred to below as ‘Conventional technology 3’).
As shown in FIG. 23, this optical scanner is provided with a plate-shaped micro mirror 121 which is used to reflect light, a pair of rotation supporting bodies 122 which are positioned on a straight line and support both sides of the micro mirror out 121, a frame portion 123 to which the pair of rotation supporting bodies 122 are connected and which surrounds the periphery of the mirror 1, and a piezoelectric element 124 which applies translational motion to the frame portion 123. In addition, this optical scanner is structured such that the center of gravity of the mirror 121 is located at a position outside the straight line connecting together the pair of rotation supporting bodies 122.
When voltage is applied to the piezoelectric element 124, the piezoelectric element 124 is made to expand and contract, so as to vibrate in the Z axial direction. This vibration is transmitted to the frame portion 123. When the micro mirror up 121 is made to undergo relative motion relative to the driven frame portion 123 and the vibration component in the Z axial direction is transmitted to the micro mirror 121, because the micro mirror out 121 has a left-right asymmetrical mass component relative to the axis formed by the X axis rotation supporting bodies 122, rotational moment is generated in the micro mirror 121 centered on the X axis rotation supporting bodies 122. In this manner, the translational motion which has been applied to the frame portion 123 by the piezoelectric element 124 is transformed into rotational motion centering on the X axis rotation supporting bodies 122 of the micro mirror 121.
Moreover, as shown in FIG. 24, the optical scanner described in Japanese Unexamined Patent Application, First Publication No. H09-197334 (Patent document 4, referred to below as ‘Conventional technology 4’) has a vibrating portion 131 which has a mirror surface on one surface thereof, a fixed portion 132 to which vibration is applied, and an elastic deformation portion 133 which links the vibrating portion 131 to the fixed portion 132 and which elastically deforms, and a spring constant variable element 134 which adjusts resonance characteristics is provided in the elastic deformation portion 133.
An electrical resistance element which is a heat-generating source or a piezoelectric element which is a distortion generating source is used for this spring constant variable element 134. The spring constant of the elastic deformation portion 133 is changed either by a deformation or changes in the temperature of the elastic deformation portion 133, so that it is possible to adjust the resonance characteristics of the vibration.
Moreover, as shown in FIG. 25, an optical scanning device is also described in Japanese Unexamined Patent Application, First Publication No. H10-104543 (Patent document 5, referred to below as ‘Conventional technology 5’). In this optical scanning device, beam portions 143 and 143 extend in mutually opposite directions from both sides of a movable portion 142 in a vibrator 141, and are connected to two arm portions 144 and 144 of a fixed portion 146. Piezoelectric thin films 145 and 145 are provided respectively on the arm portions 144 and 144 of the fixed portion 146, and these piezoelectric thin films 145 and 145 are driven by the same signal which includes higher order vibration frequencies.
In the above-described optical scanners of Conventional technologies 1, 2, 3, and 4, the properties (i.e., the optical scan angle, the optical scanning speed, and the optical scan trajectory and the like) of optical scanners which use this type of resonance vibration of a vibrator are hugely dependent on the resonance characteristics of the vibrator. Of the resonance characteristics of a vibrator, the resonance frequency, phase, and amplitude, in particular, have a considerable effect on the optical scan angle and the trajectory of the optical scan line of the optical beams emitted from the optical scanner.
If, for example, the spring constant of the elastic deformation portion (i.e., the torsion bar) in Conventional technology 4 is taken as k, and the moment around the rotation axis (i.e., the Y axis or the Z axis) is taken as I, then the resonance frequency f in a vibrator 1 can be expressed by the following formula.
                    f        =                              1                          2              ⁢              π                                ⁢                                    k              I                                                          (        1        )            
The spring constant in the bending deformation mode (in a θB direction) of the elastic deformation portion is taken as kB, while the spring constant in the torsion deformation mode (in a θT direction) is taken as kT. If the spring constant k in Formula (1) is replaced by these spring constants kB and kT, then Formula (1) shows the resonance frequency fB in the bending deformation mode, and shows the resonance frequency fT in the torsion deformation mode, and the spring constant kB in the bending deformation mode is expressed by the following formula.
                              K          B                =                              E            ⁢                                                  ⁢                          wt              3                                            4            ⁢            L                                              (        2        )            
Here, E is Young's modulus, w is the width (i.e., the length in the Y direction) of the elastic deformation potion, t is the thickness (i.e., the length in the X direction) of the elastic deformation portion, and L is the length (i.e., the length in the Z direction of the elastic deformation potion.
The spring constant kT in the torsion deformation mode is expressed by the following formula.
                              K          T                =                                            G              ⁢                                                          ⁢              β              ⁢                                                          ⁢                              wt                3                                                    12              ⁢              L                                ⁢                                          ⁢                      (                          t              <              w                        )                                              (        3        )            
Here, G is the modulus of transverse elasticity, and β is a coefficient relating to the shape of the cross section. In Formula (3), more typically, w represents the length of a long side of the cross section of the elastic deformation portion, and t represents the length of a short side of the same cross section.
It is understood from Formula (1) that, as a result of the spring constant k changing, the resonance frequency of the vibrator is changed. Moreover, the Young's modulus E in Formula (2) and the modulus of transverse elasticity G in Formula (3) are also known as material constants, and because the interatomic force and shape of the elastic deformation portion are changed by thermal expansion in accordance with changes in the external temperature environment, these material constants also change.
Accordingly, if the temperature of the operating environment of an optical scanner changes, or if the temperature of the optical scanner itself rises because of heat generated from a drive source thereof, the resonance frequency also changes. Because of this, if the frequency of the drive source is fixed, the scan angle of the mirror becomes smaller and it is not possible for it to be kept constant. The result of this has been that it has been difficult for these optical scanners to be used in display devices or precision measuring instruments which are used in real environments because more the value Q (quality factor value, Q=ω0 m/r, m: mass, r: resistance) of the mechanical resonance system of the optical scanner is enlarged in order to lower the drive voltage of the optical scanner or to increase the scan angle of the mirror, then the more the change in the mirror scan angle due to the environmental temperature becomes sharper and larger.
Moreover, in a process (for example, silicon etching, metal etching, and the like) to manufacture a vibrator in which a vibrator is manufactured by processing a normal silicon substrate and metal, irregularities occur easily in the shape of the vibrator during processing. Irregularities in the shape of the vibrator cause irregularities in the resonance characteristics of the vibrator.
Adjusting the resonance characteristics of a vibrator by adding weight to a specific position on the vibrator, or by forming comb teeth in advance on portions of the vibrator and then removing these comb teeth one by one may be considered, however, there are problems in that irregularities easily occur in the positions where weight is attached, a microfabrication is required in order to form the comb teeth, or the like. It is also easy for deformation and breakage to occur in the vibrator as a result of these adjustments, and it becomes difficult for adjustments of the resonance characteristics to be performed repeatedly. Above all, when adding weight or removing comb teeth, the amount of adjustment is too large to allow precise adjustments to be made either easily or at all.
Because these types of problems exist, in order to keep the optical scanning performance (i.e., the resonance frequency, the mirror scan angle, and the phase) of an optical scanner device uniform in the face of changes in the surrounding environmental temperature and in the face of irregularities in manufacturing, it is necessary to correct the circuit constant in the optical scanner drive circuit and photoreceptor signal processing circuit. The increased costs brought about by these adjustments are a big problem in practical applications.
In the conventionally known prior technologies 1, 2, 3, and 4, the resonance frequency f of the torsional vibration of a vibrator whose mirror portion is supported by a torsion bar is determined by the weight of the mirror portion (in this case, the rotational moment I of the mirror portion) and the length L of the torsion bar, and by the spring constant Kt in the torsion direction of the torsion bar which are expressed in the above-described Formulas (1) and (3). In contrast to this, as shown in PART (a) of FIG. 1, the resonance frequency of the torsional vibration of the mirror portion of the optical scanning device in which plate waves or vibration are used and which is the subject of the invention are not determined solely by the weight of the mirror portion (in this case, the rotational moment I of the mirror portion) and the length L of the torsion bar, and by the spring constant Kt in the torsion direction of the torsion bar, but are also affected considerably by the shape and size as well as the thickness of the substrate (i.e., the frame structural portion) itself by which the mirror portion and portion bar are linked and supported, and by the spring constant Kf. FIGS. 1 and 2 illustrate this difference by means of a simulation based on the finite element method. The optical scanning devices shown in FIGS. 1 (a) and 1 (b) PART (a) and PART (b) of FIG. 1 both have mirror portions and torsion bars having exactly the same shape and mechanical characteristics, however, in the optical scanning device shown in PART (b) of FIG. 1, the thickness of the substrate portion (i.e., the frame structural portion) supporting the cantilevers which support the mirror portion and torsion bars is twice that of the example shown in PART (a) of FIG. 1, so that the spring constant Kf (i.e., the rigidity) thereof is increased. A comparison of the resonance frequencies f and scan angles θ of these two is shown in FIG. 2. The above-described resonance frequencies have considerably shifted to the high frequency side.
Accordingly, by changing not only the spring constant Kt in the torsion direction of the torsion bars and the shape (i.e., the cross-sectional configuration and the length L) of the torsion bars themselves, but by also changing the spring constant Kf and the shape of the substrate (i.e. the frame structural portion) itself by which the mirror portion and torsion bars are joined and supported, it is possible to change the resonance frequency f of the torsional vibration of a vibrator whose mirror portion is supported by torsion bars.
Conversely, if the surrounding environmental temperature of the optical scanning device is changed, then the spring constant Kt in the torsion direction of the torsion bars and the shape (i.e., the cross-sectional configuration and the length L) of the torsion bars themselves, or the spring constant Kf and the shape of the substrate (i.e. the frame structural portion) itself by which the mirror portion and torsion bars are joined and supported are also changed, and the resonance frequency f of the torsional vibration of a vibrator whose mirror portion is supported by torsion bars is changed.
Moreover, the optical scanning device of the above-described Conventional technology 5 has the drawback that a large torsion angle cannot be formed in the movable portion 142.
Namely, if a piezoelectric film is formed in the two narrow cantilever beam portions which support the two torsion bars protruding from the frame portion, then the rigidity of this portion increases and vibration which is induced in the piezoelectric film is not transmitted efficiently to the torsion bars. As a result, the torsion vibration of the mirror is reduced. Moreover, unless the vibration characteristics of the vibration source portion formed by the two cantilever beam portions and the piezoelectric film which is formed thereon are matched precisely, then the vibration amplitude of the torsional vibration of the mirror becomes suppressed and, at the same time as this, torsion modes other than torsional vibration are accumulated so that accurate laser beam scanning cannot be achieved. Furthermore, in order to increase the drive power for the mirror by increasing the surface area of the piezoelectric film portion, it is necessary to increase the width of the cantilever beam portions. Because of this, an unnecessary two-dimensional vibration mode is generated in the same cantilever beam portion, so that at the same time as the vibration amplitude of the torsional vibration of the mirror is restricted, a vibration mode other than the torsional vibration is superimposed thereon. As a result, the problem arises that it is not possible to achieve accurate laser beam scanning. Moreover, because the width of the cantilever beams is restricted to a narrow width, the formation of the top portion electrodes which are used to drive the piezoelectric film formed on this portion is made more difficult because of the narrow width, so that problems arise such as the yield during production being greatly affected.
FIG. 26 shows the same case as that of the Conventional technology 5, and shows a structure in which a piezoelectric film is formed on two narrow-width cantilever beam portions which support two torsion bars which protrude from a frame portion. The drive efficiency of the mirror portion scan angle was checked by a simulation calculation. The surface where Y=0 was taken as a plane of symmetry, and half of this was used as a model.
FIG. 27 shows the torsion angle of a mirror having a structure in which a piezoelectric film is formed on two narrow-width cantilever beam portions which support two torsion bars which protrude from the frame portion shown in FIG. 23. The drive voltage was set at 1 V, while the characteristics of a PZT-5A which are typical parameters were used for the electrical characteristics of the piezoelectric body, while SUS 304 characteristics were used for the material of the scanner frame main body. The torsion angle of the mirror portion was small at only 0.63°.